1. Field of the Invention
The invention relates to a working machine according to the preamble of patent claim 1. The invention relates, in particular, to a tamping machine for soil compaction or to a hammer.
2. Description of the Related Art
Known tamping machines of this type are designed in such a way that an upper mass receiving a motor and a crank mechanism is connected via a spring assembly to a working mass which forms essentially a working or compacting plate. The rotational movement generated by the motor is converted by the crank mechanism into an oscillating axial movement which is transmitted via the spring assembly to the working plate for soil compaction. The upper mass comprises about two thirds and the percussive working mass one third of the entire tamper mass, whilst the distances covered in each case by the upper mass and the working mass are in inverse proportion to one another. The order of magnitude in which the upper mass moves in this case is 25 to 30 mm.
The vibrations of the upper mass are transmitted via a guide handle to the person guiding the working machine, and is very unpleasant, particularly when the work lasts a relatively long time. In this context, vibrations in the horizontal or lateral direction are particularly troublesome for the operator. By contrast, vibrations in the vertical direction are necessary for the tamper to work efficiently.
FIG. 2 shows a known tamper of this type.
According to FIG. 2, a drive shaft 1 of the tamper is driven by a motor, not illustrated, the drive shaft driving, via a pinion 2, a crank disk 3 mounted in the tamper housing and provided with external toothing. Attached to the crank disk 3 is a crank pin 4, onto which a connecting rod 5 is placed in a rotationally movable manner. The connecting rod 5 is connected at its other end to a guide piston 7 in a rotationally movable manner by means of a piston pin 6. The guide piston 7 carries a piston guide 9 formed by a steel disk and fastened by means of a nut 8. The guide piston 7 is movable axially back and forth, by means of the piston guide 9 within a guide tube 10 belonging to the lower mass. This axial direction corresponds to a vertical or working direction of the machine when it is being used.
A spring assembly 11 consisting of a plurality of springs is arranged on both sides of the piston guide 9, the springs in each case being supported, on their side facing away from the piston guide 9, against spring plates 12 fastened to the guide tube 10. In order to avoid the spring assemblies 11 being blocked together, a dampening bush 13 made from an elastic plastic is placed onto the guide piston 7 above the piston guide 9, whilst a damping plug 14, likewise consisting of elastic plastic, is attached below the nut 8. When the spring assemblies 11 are highly compressed, the dampening bush 13 and the damping plug 14 can in each case butt onto the associated spring plate 12 with their side facing away from the piston guide 9. They then damp the further compressive movement in such a way that the situation can be avoided where the spring assemblies 11 are blocked together and an excessive impact action is consequently exerted on the working machine.
The guide tube 10, together with the spring plates 12, belongs to the working or lower mass of the tamper. A tamping foot, not shown in FIG. 1, which serves for soil compaction may be attached to the lower mass. In order to avoid the penetration of moisture and dirt, the upper mass and the lower mass are connected by means of an elastic concertina 15.
As is apparent from FIG. 2, the rotational movement of the motor is converted into an oscillating axial movement of the guide piston 7 by the crank mechanism by means of the crank disk 3, the crank pin 4 and the connecting rod 5. This axial movement is transmitted via the spring assemblies 11 to the guide tube 10 and consequently to the lower mass and can be utilized for soil compaction.
In order to damp the vibrations acting on the operator, it has been known hitherto to uncouple the guide handle from the upper mass mechanically by means of rubber elements. In this case, however, the mounted drive motor still remains exposed to high vibrational loads. An improvement in vibration damping can be achieved here only at a high outlay in terms of construction.
It is therefore desirable, from the outset, to avoid vibrations of the upper mass occurring.
DE-A 19 25 870 discloses a tamper for soil compaction, with a working mass which is driven linearly back and forth, via a double crank mechanism, by a motor belonging to an upper mass. In order to reduce the vibrations on the upper mass, two weights moveable in opposition are provided, which superpose an oppositely directed vibration on the vibration generated by the crank mechanism. The tamper has a double-leg design, each tamper leg being driven via its own crank mechanism. The tamper correspondingly has a very large build and can be guided on the ground only with great effort.
DE-Patent 753 502 discloses a drive device for exciting vibratory systems. For this purpose, arms and levers coupled to one another via rubber springs are provided in a crank mechanism. In order to avoid harmful dynamic mass action in the form of forces reacting on the motor and the bearings, the mass of the arms and levers is kept as low as possible, using materials of low specific gravity.